Nuclear proliferation represents a global and imminent threat to the safety and security of the international community. Smuggling of contraband radioactive materials can be counteracted with effective neutron detection technologies deployed along shipping routes and at potential terrorist target locations. However, established technologies commonly utilize helium-3, which is a rare isotope of which in recent years demand has overtaken supply. Thus, the production of effective, inexpensive, and widely deployable neutron detectors is a high priority. Liquid and/or plastic scintillating detectors are a promising prospect for inexpensive neutron detectors.
A scintillator is a material that exhibits scintillation (e.g., the property of luminescence) when excited by ionizing radiation. Sometimes, the excited state is metastable, so the relaxation back out of the excited state is delayed (necessitating anywhere from a few microseconds to hours depending on the material): the process then corresponds to either one of two phenomena, depending on the type of transition and hence the wavelength of the emitted optical photon: delayed fluorescence or phosphorescence, also called after-glow.
Atomic and subatomic particles are detected by the signature they produce through interaction with their surroundings. The interactions result from the particles' fundamental characteristics. Neutron detection may be achieved by elastic scattering reactions (also referred to as proton-recoil), where high energy neutrons may be detected indirectly through elastic scattering reactions. In particular, in response to radiation, as a neutron is not charged it does not interact via the Coulomb force (e.g., electrostatic interactions between electrically charged particles) and therefore does not interact with the electrons in a scintillation material, but when neutrons collide with the nucleus of atoms in a detector, the collision transfers energy to that nucleus and creates an ion (e.g., ionization), which is detectable. Kinematically, a neutron can transfer more energy to light nuclei such as hydrogen or helium than to heavier nuclei. Detectors relying on elastic scattering are called fast neutron detectors. Recoiling nuclei can ionize and excite further atoms through collisions. Charge and/or scintillation light produced in this way can be collected to produce a detected signal.
Thus, fast neutrons (e.g., generally >0.5 MeV) primarily rely on the recoil proton in a (n,p) scattering reaction and materials rich in hydrogen (e.g. liquid and plastic scintillators) are therefore well-suited for their detection. Slow neutrons rely on nuclear reactions such as the (n,γ) or (n,α) reactions, to produce ionization. Their mean free path is therefore quite large unless the scintillator material contains nuclides having a high cross section for these nuclear reactions, such as 6Li or 10B. Materials such as LiI(Eu) or glass silicates are therefore well-suited for the detection of slow (thermal) neutrons.
Fast neutron as well as thermalized neutron detection is a difficult undertaking but necessary in basic nuclear science as well as in the applied fields of nuclear energy, nuclear safeguards, nuclear forensics and homeland security. Neutrons interact differently with commonly used detector materials compared to, for example, gamma radiation, which is usually also prevalent in neutron radiation fields. Therefore, efficient neutron detection usually requires large detectors which need to employ a mechanism to distinguish the neutron and gamma radiation.
The term “liquid scintillator” typically refers to a liquid solution of one or more organic scintillators in a solvent. The term “plastic scintillator” typically refers to a scintillating material in which the primary fluorescent emitter, called a fluor, is suspended in the base, a solid polymer matrix.
Scintillators may be used as neutron (e.g., radiation) detectors, in neutron and high energy particle physics experiments, in new energy resource exploration, in nuclear cameras, for computed tomography, and for gas exploration, among other uses. In particular, a scintillator may be used in conjunction with a photomultiplier tube (“PMT”).
PMTs absorb the light emitted by the scintillator and reemit it in the form of electrons via the photoelectric effect. The subsequent multiplication of those electrons (sometimes called photo-electrons) results in an electrical pulse which can then be analyzed and yield meaningful information about the particle that originally struck the scintillator. Photomultiplier tubes (e.g., vacuum tubes or vacuum phototubes) are extremely sensitive detectors of light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum. Thus, scintillators in combination with a PMT are useful in survey meters used for detecting and measuring radioactive contamination and monitoring nuclear material. However, detection in scintillators is based on the collection of luminescence emitted by the scintillator components when they interact with particles of nuclear origins. Therefore, to maximize light collection, scintillator transparency is desired.
In addition, natural boron or 10B loaded scintillators are costly. Therefore, scintillators that may be produced at lower cost, including any that may be produced using different compounds to reduce cost and improve ease of manufacture, are desired.